To realize an information recording medium (such as an optical disk) of a great recording capacity, multi-layered (or ultra-multi-layered) information recording media with a large number of stacked data layers have recently been developed and put into practical use. On the other hand, reduction of productivity of information recording media caused by increase of data layers, in particular, degradation of production yield and increase of manufacturing tact time have become problematic. One of the processes that may cause the problems is a process of forming, in each layer of an information recording medium, a tracking servo layer and a spiral track groove (which will be also referred to simply as “groove”) serving as a guide for address designation. For instance, in the case of an optical disk with ten layers, it is necessary to execute, in each layer, ten shape-forming steps using different stampers. This requires a process time, and even a single defective layer, if any, makes the disk itself defective.
In view of this, an information recording medium of a guide layer type, in which a plurality of data recording layers and a servo layer for tracking servo are provided separately, has been proposed. More specifically, a technique of providing one servo layer (SL) and four data layers (DL) in one optical disk has been proposed. In this technique, only the servo layer has a groove, and the data layers are formed flat. In the case of the information recording medium having the servo layer and the data layers separate from the former, it is sufficient if the shape-forming process is performed only on the servo layer. Therefore, this technique is advantageous in that even if the number of layers is increased to realize a large capacity, the number of shape forming steps does not increase.
However, in the information recording medium of the guide layer type, data for discriminating layer numbers cannot be beforehand recorded on the data layers because the data layers are formed so as not to have an uneven structure. Accordingly, at the time of recording/reproducing, it cannot be determined what number of data layer is currently being focused by a recording/reproducing beam.
As a method of recording information for control or management on an information recording medium after forming the information recording medium, a method utilizing BCA (Burst Cutting Area) information as in, for example, a DVD is already known. However, when the known method of recording BCA information is naively applied to an information recording medium of the guide layer type, significant leakage of signals between adjacent layers may well occur and significantly degrade BCA reproducing signals, which makes it difficult to record different BCA information items in a plurality of layers.
On the other hand, there is a technique of providing focus servo setting areas of different reflectance ratios along the radius of a disk. In this technique, it is possible to enhance the probability of success of a shifting operation to a desired focus position. However, since there is no means for determining, after the completion of the setting operation, what number of data layer in the data layers is the actually focused data layer, it cannot accurately be determined whether the focus servo setting operation has succeeded.